1. Field of the Invention
The present invention relates to a dithering method that can be applied to image processing by an image processing apparatus. Particularly, the present invention relates to a dithering method that is applied to quantization processing by an image processing apparatus that forms an image on a printing medium by ejecting ink from ejection openings.
2. Description of the Related Art
In an inkjet printing apparatus as a kind of image processing apparatus, printing data to be printed by the printing apparatus is generated by performing image processing such as quantization processing of image data that is inputted to the printing apparatus.
In this type of printing apparatus, in order to reduce the granular feeling of an image, the dot arrangement on the printing medium or the number of times ink is ejected may be set according to the size of the ink drops that are ejected from the ejection openings or according to the density of the ink. Moreover, in a color printing apparatus that uses plural colors of ink, the dot arrangement on the printing medium or the number of times ink is ejected may be set by taking into consideration the usage ratio of each ink color, the coverage of dots on the printing medium, or the overlapping of dots.
Japanese Patent Laid-Open No. 2009-177852 discloses an image processing apparatus that uses an error diffusion processing method for quantization processing. In this image processing apparatus, cyan dots and magenta dots are not printed in areas where both the density value of the cyan component and the density value of the magenta component indicate low density. On the other hand, cyan and magenta dots are independently printed in areas where both of the density values indicate intermediate to high density, and dots of the color having the largest density value are exclusively printed in areas where only one of the density values indicates intermediate to high density.
In the case of the image processing apparatus disclosed in Japanese Patent Laid-Open No. 2009-177852, when performing quantization processing of the cyan color component data and when performing the quantization processing of the magenta color component data in this way, the dot arrangement of each color is determined by comparing the density value of the cyan component with the density value of the magenta component.
Moreover, U.S. Pat. No. 6,867,884 discloses a method that uses a dithering method for the quantization processing. In this method, a single dither mask is used in common for black, cyan, magenta and yellow.
In the method disclosed in U.S. Pat. No. 6,867,884, threshold values inside the dither mask are taken to be the dot arrangement range, and the dot arrangements used are assigned in order for the input gradation values for each color. Then, the printed dots (1) or non-printed dots (0) are set by comparing the threshold values and input gradation values that were newly set for the respective assigned dot arrangements. As a result, in the method disclosed in U.S. Pat. No. 6,867,884, black, cyan and magenta dots are arranged without overlapping.
However, in the image processing apparatus disclosed in Japanese Patent Laid-Open No. 2009-177852, error diffusion processing is performed while referencing the density values of other ink color components, so the processing load in error diffusion processing that originally has a large processing load is further increased. As a result, when performing the error diffusion processing using hardware, the comparator, subtracter, memory for temporarily storing error, and the like are also increased, which brings about an increase in cost of the apparatus. Moreover, when the error diffusion processing is performed using software, the load on the CPU is increased, so there is a problem in that the processing speed and printing speed are decreased.
In the method disclosed in U.S. Pat. No. 6,867,884, a single dither mask is used in common for each color, so even though it is possible to disperse the dot arrangement for plural ink colors on a printing medium without overlap, arranging the dots with no correlation is difficult. As a result, when an error such as a conveying error occurs, shifting between dot groups of each ink color in areas where the ink density is dense causes the coverage of dots on the printing medium to fluctuate, so there is a possibility that irregular density will occur.